Optical disk recorders, in particular, employ objective lenses having very small size and mass. Such lenses are typically moveable along their optical axis for effecting focussing motions. The depth of field of such lenses is typically small such that an in-focus condition must be rather precise for effecting effective recording and readback of signals onto and from an optical disk. Generally, the acquisition of focus of the objective lens, i.e., the movement of the objective lens to an in-focus position, should be reliably and quickly accomplished for preventing retries and other delays in using an optical disk recorder. The initially acquired focus may not be a best focus for use in a particular application. For example, in video recorders, it is known that the amplitude of the video readback signal is used for obtaining a best focus indication. In many optical disk of the data recording type, there is no recorded signal for effecting the calibration of focus to a maximum readback amplitude.
Further, focussing errors in an optical disk drive may result in a spot of focussed light that is larger than desired which produces a smaller amplitude readback signal and less light intensity at the disk during write operations. Such conditions can cause errors in data recording and readback. Offset errors in the focus error signal (FES) may be caused by initial manufacturing tolerances as well as thermal effects. It is desired to eliminate these steady state errors.
Many focus acquisition systems find the peak of the FES signal which is adjacent to a idealized focus condition, as is well known. Finding the peak may require measurement of a large number of points on the FES signal; this action is particularly true when there are minor fluctuations occurring over the range of the FES signal with respect to changing focus conditions. Such an action is time consuming since for each focus offset value an entire rotation of the disk would normally be required for eliminating signal variations due to disk angular positions. Other considerations in focussing systems is that the peak of the FES signal that represents best focus may not be the most desirable operating point. Machine vibration as well as disk runout may produce small errors in the ability of the focussing servo to maintain the idealized in-focus condition. Under some circumstances, a slight defocussing may result in an undesired loss of readback signal amplitude. It is desired to eliminate these problems by a simple focus acquisition system.